Currently in modern medical practice there is a growing use of various prosthetic devices which are related to bone. Each year thousands of hip, knee, and other joints, which have been damaged by trauma, osteoarthritis, osteoporosis, etc., are being replaced by prosthetic devices. Additionally, there are thousands of dental procedures being employed which involve prosthesis. All of these prosthetic devices have a common element if they are to be successful in benefiting the patient, i.e., they are all anchored to bones and must remain firmly attached in order to work properly.
Great success has been achieved over the last decade or so, in the use prosthesis to replace damaged joints such knees and hips. Today, it is becoming a very common procedure, benefiting thousand of people who can return to a normal life. The economic impact of these therapies is substantial in that they reduce the need of long term custodial care and allow many people to return to their productive lives. Much of the success of these therapies is due to improved surgical methods and particularly to improvements of materials which are bio-compatible with the bone which they are anchored to. The initial success rate for the implantation of these prosthesis is very high. Most patients achieve a secure bond between the new prosthesis and the existing bone surrounding it. This bond was thought to be sufficient for the life of the individual. However, since the introduction of these therapies, it has now become apparent that in many cases the bone-prosthesis bond has a finite life and that after a period of time, this bond becomes weaker leading to ultimate failure. This problem is only now becoming increasingly important due to the fact that the expected life of the bone-prosthesis bond appears to be about 10-20 years in most individuals. Therefore, it would seem reasonable that in following years, the medical profession will see increasing numbers of patients presenting with bone prosthesis which beginning to fail, i.e., to become detached from their bone support. Also, a trend which will certainly exacerbate this problem is the use of prosthesis in younger patients. This trend of increasingly younger patients undergoing prosthetic replacement is in large part due to the excellent recovery rate of the technique and to changes in life-style such as increasing sports injuries. It would seem reasonable to expect that a patient of age thirty-five might need to have a knee prosthesis replaced two to four times in their life.
With the increasing numbers of prosthesis failures bound to rise, the orthopedic surgeon faces a daunting task. When bone prosthesis fails, it does not simply mean that the prosthesis becomes totally detached from its bone support. The most likely case is that a relatively small portion loses its structural integrity, causing pain and ultimately a fracture break. Often this fracture destroys part of the surrounding bone which was not involved with the initial bone-prosthesis separation. The orthopedic surgeon must remove the old prosthesis and replace it making sure that the new one will be firmly anchored to the supporting bone. This is where the greatest problem occurs. In the removal of the old device inevitably a portion of the supporting bone around the prosthesis is either damaged or must be removed with the old device, which means there is less good, unaffected, supporting bone left for the attachment of the new one. If there is a prospect that a given prosthesis may have to be replaced more than once, there may be little bone left for such an attachment. This situation may even occur on the first replacement.
The reason for the failure of the bone-prosthesis bond over time is not well understood. Theories as to its cause have appeared in the literature. One theory suggests that micro-movement between the supporting bone and the prosthesis induce a focal inflammation leading to bone resorption and failure. Another theory suggests that the prosthesis induces rapid turn-over bone in the proximal bone with an increase both of formation and resorption, but resorption being slightly enhanced thus leading to a net resorption over time. As yet, there is no known way to inhibit this ultimate failure of the prosthesis and the situation will become increasingly critical.
As mentioned earlier, another area of the use of prosthesis which are attached to bone is that of dentistry. Today, thousands of people have dental implants into the jaw to anchor single teeth to entire sets of teeth. The same sequelae as with joint replacement is applicable. The long term results of these implants is often the same.
Clearly, it would of great and increasingly important value to have an agent which prolongs the useful life of bone prosthesis.